4 Answers
4

a- The Mechaber (S.A. O.C. 24:5) brings that when you look at your tzitzis, you should look at the two front ones which has 10 knots alluding to the ten sefiros, and 10 knots + 16 strings = 26, alluding to the shem havaya. In 61:25 he says to handle the 2 front ones when saying urisem oso as well.

b- The proper way to wear tzitzis is when 2 corners are in front and 2 in back. (Based on M.B 10:36- although in our case it wouldn't disqualify the talis by bringing the back 2 in front since its possible to wear 2 in the back.)

In this great book, "Why we Pray what we Pray" the author makes note that the passage for Tizizit in the Shema was used to replace the reading of the 10 commandments.

The reason that the passage for tzizit was use are for the following reasons.

The reason we stopped reading the 10 commandments was because of people who wanted to argue that only those 10 mitzvot are required, and not all 613. The tzizit give us a reminder of all 613 mitzvot so it's a rebuttal.

During the section of the 10 commandments it mentions the sapphires that the Jewish people saw below the "Kisai Kavod". The tzizit with techelet remind the person of the ocean, which reminds them of the heavens which reminds them of the "Kisai Kavod"

Thus the passage of Tzizit is a good short (instead of the passage of Bilam, which proves that Gd not Moshe wrote the Torah) replacement for the reading of the 10 commandments.

So to answer your question. We kiss the front two tzizit during Baruch Shemar since they remind us of the Kisai HaKavod and the mitzvot which are a testament to Gds works which we praise in Baruch sheamar. We use the two front tzizit since they act as a pair of Aidim (witnesses) to what we are declaring.

The שו״ת זכרון
יהודה או״ח סי׳ ט׳ explains that the reason for taking two tzitzis (as opposed to all four) is because ברוך שאמר mentions the word ברוך ten times corresponding to the עשרת הדברות, and therefore we take two tzitzis which together have ten knots.

The Mishna Brura OC 51 sk 1 says to hold the front two tzitzit while saying Baruch SheAmar and kiss them at the end. His source is the Magen Avraham there sk 1 who brings it from the Arizal. I cannot comment regarding the kabbalistic significance of the action.

However, it is important to note that the Magen Avraham just mentions holding the tzitzit during not kissing them at the end.

Thus, it seems to me that maybe the main point is holding them during (for a reason such as you mentioned) and the kissing at the end is just something respectful to do before putting them down.

The explanation you gave for kissing them before you put them down makes sense, but I think we should do that after the first couple of lines, not at the end.
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Ari AJan 30 '12 at 23:37

@AriA Apparently it is kabbalistically significant to hold them the whole time. I am not a kabbalist. Maybe someone who is one will tell you how exactly that works. I'm just showing you the sources in the halacha.
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Double AA♦Jan 30 '12 at 23:40